It’s also interesting to note that NULL values are not regarded in unique indices. So, while a duplicate non-NULL value is not allowed in a column that has a unique index on it, more than one NULL value *is* allowed. I’m not sure if this behavior is consistent across all major ...
root@host# mysql -u root -p password;Enterpassword:***mysql>useRUNOOB;Databasechangedmysql>createtablerunoob_test_tbl->(->runoob_authorvarchar(40)NOTNULL, ->runoob_countINT->);QueryOK,0rowsaffected(0.05sec)mysql>INSERTINTOrunoob_test_tbl(runoob_author,runoob_count)values('RUNOOB',20);mysql>...
-> runoob_count INT -> ); Query OK, 0 rows affected (0.05 sec) mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values ('RUNOOB', 20); mysql> INSERT INTO runoob_test
ageIN(SELECTageFROMVALUES(50), (null) sub(age)); name age--- ---Fred 50 Dan 50-- Since subquery has `NULL` value in the result set, the `NOT IN`-- predicate would return UNKNOWN. Hence, no rows are-- qualified for this query.>SELECT*FROMpersonWHEREageNOTIN(SELECTageFROMVALUES(...
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obclient>CREATETABLEt_null(id NUMBERNOTNULLPRIMARYKEY,nameVARCHAR(10));Query OK,0rowsaffected obclient>INSERTINTOt_null(id,name)VALUES(1,'A'),(2,NULL),(3,'NULL');Query OK,3rowsaffected Records:3Duplicates:0Warnings:0obclient>SELECTid,name,NVL(name,'NOT APPLICABLE')n_name,IFNULL(name,...
下面向子查询的t2中插入一条c2列为null的记录。greatsql> insertinto t2 values(,null);再观察一下三条语句的执行结果:greatsql> select * from t1 where t1.c2 notin (select t2.c2 from t2);Empt 结论 使用not in 的非关联子查询注意NULL值对结果集的影响,为避免出现空结果集,需要子查询中查询列加is...
greatsql> create table t1 (id int, name varchar(30), age int, addr varchar(30), school varchar(30), unique key (id)) engine=innodb; greatsql> insert into t1 values (1,'小红',10,'北京','一中'), (2,'小绿',11,'北京','一中'), ...
values(4,null,1); mysql> select * from test_ab; +---+---+---+ | id | col_a | col_b | +---+---+---+ | 1 | 1 | 1 | | 2 | | | | 3 | NULL | | | 4 | NULL | 1 | +---+---+---+ 4 rows in set (0.00 sec) 首先比较一下,空字符('')和空值(null)查...
mysql>insertintotest(id , name)values(1,null); Query OK,1rowaffected (0.01sec) mysql>insertintotest(id , name)values(2,''); Query OK,1rowaffected (0.01sec) mysql>insertintotest(id , name)values(3,' '); Query OK,1rowaffected (0.00sec) ...